1. Field of the Invention
The present invention relates to colour electrostatography in general, and more particularly to a device for improving transfer of successive, different colour toner images in superimposed registration with one another on a receptor sheet.
2. Description of the Prior Art
In an electrophotographic printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is image-wise exposed. Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas. As a result, an electrostatic latent image is recorded on the photoconductive member corresponding to the informational areas contained in the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing toner into contact therewith. This forms a developed toner image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multicolour electrophotographic printing is substantially identical to black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colours are recorded thereon. Each single colour electrostatic latent image is developed with toner of a colour complementary thereto. This process is repeated a plurality of cycles for differently coloured images and their respective complementarily coloured toner. Each single colour toner image is transferred to the copy sheet in superimposed registration with the prior toner image, thereby creating a multilayered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently fixed to the receptor sheet creating a colour copy or print. The developer material may be a liquid material or a powder material.
In order to successfully transfer different colour toner images to the sheet, the sheet can move in a path enabling successive different colour images to be transferred thereto. In this way the different colour toner images (e.g. magenta, cyan, yellow and black toner images) are transferred to the sheet. A corona generator may be used to charge the sheet to attract the toner images thereto. However, there is a significant reduction in transfer efficiency when attempting to transfer toner images to a location on the sheet having a toner image previously transferred thereto. Moreover, when more than one colour toner is used to develop a line, the transferred line is blurred. Also, transfer may be mottled and non-uniform.
One device for improving transfer of toner images in colour electrophotography is disclosed in U.S. Pat. No. 5,059,990. This device comprises one corona generator for applying a charge to the sheet of a polarity opposite to that of the charge on the toner image for transferring the toner image from the member to the sheet, another corona generator located after the first one and in the direction of movement of the sheet, for applying a charge to the sheet of a polarity which is the same as that of the charge on the toner image to assist in separating the sheet from the member, still another corona generator for applying a charge on the transferred toner image of a polarity opposite to that of the toner image, and means for electrically grounding the sheet as the latter corona generator applies the charge on the toner. The latter corona has a neutralizing effect on the transferred toner image.
We have found that this system has the disadvantage that in micro areas where unusually large amounts of toner are transferred, small amounts of toner on the sheet will get a reverse polarity. This causes back transfer of toner to the photoconductive member in a next toner image transfer station which of course is not desired.